Security inspection is of great importance in fields such as anti-terrorism and fighting against trafficking in drugs and smuggling. After terrorist attacks of the United States on Sep. 11, 2001, countries of the whole world attach more and more importance to security inspection on civil aviation. At the same time, the requirements on the security inspection of various cargo become higher and higher with the in-depth development of fighting against trafficking in drugs and smuggling. A series of security inspection measures are taken to inspect passenger's luggage and articles and cargo containers at public sites such as airports, stations, customs house and docks.
Computed tomography (“CT” for short) technology is widely used in the field of medical diagnosis and industrial nondestructive detection. The demand thereof in public security and social security increases gradually with the development of the society. Among the widely used CT scanning systems, the circular orbit scanning manner forms the majority. Such scanning manner requires a simple mechanical structure and thus is easy to realize in engineering. Besides, the corresponding reconstruction algorithm is mature and reliable. In the circular-orbit scanning system, a fan-beam CT manner or a cone-beam CT manner is usually adopted. The corresponding detectors are respectively a linear array of detectors or a plane array of detectors. The pair of an X radiation source and a detector are disposed symmetrically with respect to the rotation center of an object rotary table.
In CT scanning of objects having a large bulk using a CT scanning system, the feasible method is generally to adopt the scanning manner of rotation plus translation. That is, the object to be inspected rotates around the center axis and the radiation source detector moves simultaneously in the direction parallel to the rotation axis, thereby forming a spiral scanning trace around the object to be inspected. As to objects having a large cross section, it needs a great number of projections if a precise reconstruction is desired. This results in that the speed of the security inspection is very slow, and the efficiency is very low. Besides, a large number of data are possibly not concerned with the user. Therefore, such CT scanning system is unpractical for the airport that needs to carry out security inspection on a great number of cargoes every day due to its slower speed.
In addition, as to perspective imaging, if there are multiple articles in the direction parallel to the beam or rays, the articles overlap one another in the image. Generally, it is very hard to distinguish the articles from one another, which brings about a good many of difficulties to the inspection of contrabands.